A. Field of the Invention
The invention relates to contaminant detection and measurement and, more particularly, to subsurface contaminant detection and measurement.
B. Prior Art
The detection and measurement of subsurface contaminants is a key factor in efforts to maintain and improve the quality of the environment. This is particularly the case with respect to drinking water, which is highly susceptible to contamination during its passage through underground streams in the water cycle.
Conventional methods for monitoring subsurface liquid contaminants include both periodic sampling, with subsequent field or laboratory analysis, and in situ measurement. The former provides a reasonable measure of contamination for many purposes, but is often unsuited to precise measurement of contaminants which may be present in low concentrations (parts per million or less) since the sampling process itself disturbs the sample and may in fact alter the concentration of the constituents of interest. The latter method provides a means of avoiding this disturbance, but requires positioning a sensor containing sensitive equipment in an environment that is frequently hostile to the longevity of the sensor and its associated elements. Further, the sensor and its related equipment are typically quite expensive, and the number of sites that can practically be monitored is correspondingly limited.
Fiber optic signal transmission systems are being used in many applications where information must be transmitted from a remote site, including ground water measurement systems. See Hirschfeld et. al., Optical Engineering, vol. 22, no.5 pp. 527-531 (1983). Such systems typically irradiate a sample with an optical signal, and transmit the resultant return signal down the cable for analysis at a remote location. This allows removal of many of the sensitive elements of the sensor from the sensor head, and thereby contributes to the longevity of the system. However, the head itself is still subject to contamination from its environment, and this can affect not only its practical life but also the quality of the measurements.